Interpretive Summary: Phosphorus (P) deficiency is a major contributor of low crop yields in a majority of soils under crops. Application of P fertilizer is an essential practice for sustainable crop production. If not properly managed, large portions of the applied P is lost from agricultural lands by leaching and erosion. Such P is carried by runoff water from the agricultural lands and can cause algal booms in receiving waters. Phosphorus in surface runoff from citrus groves is suspected to contribute to the deterioration of surface water bodies, such as in the Indian River Lagoon and Lake Okeechobee in South Florida. Objectives of this study were to find the relationship between available soil P pool, soil phosphatase activities and P loadings of surface runoff, and further determine the linkage between these parameters. Obtained results showed that surface runoff P was directly derived from soil available P pool. Findings from this study will be very helpful to fine tune the best P fertilizer management practices for citrus plantation and further provide needed information to reduce the P losses from citrus growing areas.

Technical Abstract:
Phosphorus (P) losses through surface runoff from agricultural lands have been an increasing concern, due to its being a non-point source of P contamination of fresh waters. Five representative commercial citrus groves (C1-C5) located in South Florida were studied to estimate the relationships between soil P fractions, soil phosphatase activities and P levels in surface runoff water. A modified Hedley P fractionation framework was employed to fractionate soil P. Soil P was dominated by organically-bound and Ca/Mg-bound P fractions. Organically-bound P was the predominant form in the acid sandy soils from the C2 and C3 sites, as compared with the neutral/weakly alkaline sandy soils from the C1, C4, and C5 sites. Ca/Mg bound-P accounted for 44.8-59.9% of soil total P in the C1, C4, and C5 soils, but only 4% and 7% respectively in the C2 and C3 soils. Plant-available P decreased in the order of C3 > C4 > C1 > C2 > C5, which is readily subject to surface runoff and leaching losses. Total P, inorganic-P, and total dissolved P (TDP) in surface runoff were significantly correlated with soil biological P and plant-available P forms (p<0.01), suggesting that surface runoff P was directly derived from soil available P pools. Soil neutral and original phosphatase activities were related to total P (TP), inorganic P, and TDP in surface runoff and plant-available P and biological P forms in soils. Therefore, these P fractions could serve as an index of surface runoff, P loss potential, and soil P availability. However, soil acid and alkaline phosphatases did not have significant correlations with the biological and plant-available P fractions in the soils and P concentrations in surface runoff.